The present invention relates to a coupling device for an atomic force microscope with acoustic sample excitation in accordance with the preamble of claim 1, to a corresponding atomic force microscope and to a corresponding method for sound coupling at such an atomic force microscope with acoustic sample excitation. The coupling device in accordance with the invention can in this respect in particular be used within the framework of the so-called AFAM technique (AFAM=atomic force acoustic microscopy) as well as for other apparatus and methods based on atomic force microscopy in which the acoustic excitation takes place via the sample. The present invention can in particular also be used in the UFM technique (UFM=ultrasonic force microscopy). The invention will be explained in the following with reference to the AFAM technique, a significant representative of atomic force microscopy with acoustic sample excitation.
The AFAM technique is already familiar to the skilled person (U.S. Pat. No. 5,675,075). This technique is used for the high-resolution imaging of qualitative, elastic differences or for the quantitative measurement of elastic properties of sample bodies or of samples. In this technique, a sample is acoustically coupled to an ultrasonic generator (ultrasonic test head). In this respect, the sample body is located on the sound generator, with a thin film of a coupling means (e.g. viscous glycerin, hardened silver conductive lacquer, or similar) being located between the sample and the test head. This structure makes it possible to couple longitudinal sound waves into the sample and to excite the sample surface to continuous vertical oscillations (so-called out-of-plane oscillations). These oscillations are then coupled via the sensor tip lying over it into the AFM spring hanger of the atomic force microscope. The resonance behavior of the resulting spring hanger oscillations is evaluated (e.g. position of the contact resonance frequencies) to determine elastic properties of the sample. FIG. 1 shows for this purpose the principle of the AFAM technique known from the prior art (B is the plate spring or the cantilever with the needle fastened thereto for the scanning of the sample).
In the above-described apparatus and methods as with the other apparatus and methods known from the prior art, the sample is fixedly fixed to the ultrasonic generator or the ultrasonic test head by a viscous or hardened acoustic coupling means. A substantial disadvantage thereby results such that the sample and the sound generator are not mutually laterally displaceable. The acoustic excitation of surface oscillations is therefore very locally restricted. Measurement sites on samples having a large surface such as silicon wafers or photomasks can therefore only be examined with an increased time effort (due to the required releasing and refastening of the ultrasonic test head) and/or material effort (use of a plurality of ultrasonic heads).
A further disadvantage of the methods of the prior art is that the residue-free removal of the coupling means after the measurement requires a substantial effort. The use of typical coupling means such as glycerin or silver conductive lacquer is only possible with great limitations under clean room conditions and under the assumption of a desired further use of the sample.